Systems and methods of distributed optimization

ABSTRACT

Systems and methods of determining a global model are provided. In particular, one or more local updates can be received from a plurality of user devices. Each local update can be determined by the respective user device based at least in part on one or more data examples stored on the user device. The one or more data examples stored on the plurality of user devices are distributed on an uneven basis, such that no user device includes a representative sample of the overall distribution of data examples. The local updates can then be aggregated to determine a global model.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No. 15/045,707 having a filing date of Feb. 17, 2016, which claims the benefit of U.S. Provisional Application Ser. No. 62/242,771 filed Oct. 16, 2015. Applicant claims priority to and the benefit of each of such applications. Applicant hereby incorporates U.S. application Ser. No. 15/045,707 by reference in its entirety.

FIELD

The present disclosure relates generally to distributed optimization algorithms, and more particularly to performing optimization algorithms using data distributed unevenly among a large number of computational nodes.

BACKGROUND

In recent years, machine learning has been increasingly used to solve a number of problems. The amount of data collected for use in solving such problems has also increased in recent years. As the amount of such data increases, it can become difficult to store an entire dataset at a single location. In this manner, no single computing device may have direct access to the entire dataset needed to solve a problem. Conventional training methods for solving machine learning problems in such environments can include collecting a plurality of training data examples at a centralized location (e.g. a server device) wherein the data examples can be shuffled and redistributed evenly among the computing devices.

SUMMARY

Aspects and advantages of embodiments of the present disclosure will be set forth in part in the following description, or may be learned from the description, or may be learned through practice of the embodiments.

One example aspect of the present disclosure is directed to a computer-implemented method of updating a global model based on unevenly distributed data. The method includes receiving, by one or more computing devices, one or more local updates from a plurality of user devices. Each local update is determined by the respective user device based at least in part on one or more data examples stored on the respective user device. The one or more data examples stored on the plurality of user devices are distributed on an uneven basis, such that no user device includes a representative sample of an overall distribution of data examples. The method further includes aggregating, by the one or more computing devices, the received local updates to determine a global model.

Other example aspects of the present disclosure are directed to systems, apparatus, tangible, non-transitory computer-readable media, user interfaces, memory devices, and electronic devices for solving optimization problems in distributed data environments.

These and other features, aspects and advantages of various embodiments will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the description, serve to explain the related principles.

BRIEF DESCRIPTION OF THE DRAWINGS

Detailed discussion of embodiments directed to one of ordinary skill in the art are set forth in the specification, which makes reference to the appended figures, in which:

FIG. 1 depicts an example computing system for implementing optimization techniques over a plurality of nodes according to example embodiments of the present disclosure;

FIG. 2 depicts a flow diagram of an example method of determining a global model according to example aspects of the present disclosure; and

FIG. 3 depicts an example system according to example embodiments of the present disclosure.

DETAILED DESCRIPTION

Reference now will be made in detail to embodiments, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the embodiments, not limitation of the present disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments without departing from the scope or spirit of the present disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that aspects of the present disclosure cover such modifications and variations.

Example aspects of the present disclosure are directed to solving optimization problems defined by an unevenly distributed dataset. For instance, data points used in solving an optimization problem may be stored across a plurality of devices. In this manner, the data on each device may be differently distributed, and may not be a representative sample of the overall distribution of data. For instance, each device may store different amounts of data, and/or different types of data. For instance, some data features may occur on only a small subset of devices.

In this setting, communication efficiency is important and the need to minimize rounds of communication can be a goal. In particular, communication constraints arise naturally in the massively distributed setting, as network connectivity may be limited (e.g., one may wish to defer all communication until the mobile device is charging and connected to a Wi-Fi network). Thus, in realistic scenarios the system may be limited to only a single round of communication per day. This implies that, within reasonable bounds, access to local computational power is more available than communication bandwidth. As a result, one practical objective can be to minimize the rounds of communication.

According to a particular implementation, a central computing device, such as a server or data center can provide data indicative of a current global model to the plurality of computing devices. The plurality of computing devices can determine one or more local updates to the model using data stored on the respective computing devices. For instance, in some implementations, at least one of the computing devices can be a user device, such as a wearable computing device or other user device. The data examples may be generated, for instance, through interaction of a user with the user device. In this manner, the local update can correspond to a model using data generated through use of the user device by the user. For instance, the data examples may include, without limitation, image files, video files, inputted text, or data indicative of other interactions by the user with the user device. The model may be implemented in solving one or more problems, such as predictive typing, predictive image sharing, image classification, voice recognition, next-word-prediction, and/or various other suitable problems relating to use of the user device.

Each computing device can then provide the local update(s) to the central computing device. For instance, the local update can be a gradient vector. In some implementations, the local update may be determined using one or more gradient descent techniques. For instance, the local update may be determined using batch gradient descent techniques, stochastic gradient descent techniques, or other gradient descent techniques. The local update does not include the training data used to determine the local update. In this manner, the size of the local update can be independent of the training data used to determine the local update, thereby reducing bandwidth requirements and maintaining user privacy. In particular, a global model can be updated based at least in part on the received local updates. By only providing the local update (and not the training data) to the server, the global model update can be determined using reduced bandwidth requirements, and without compromising the security of potentially privacy sensitive data stored on the user devices. In addition, through aspects of the present disclosure, the global model can be trained using a smaller number of communication rounds, and so this also reduces the network bandwidth needed for training by orders of magnitude compared to copying the data to the datacenter. In some implementations, the global model update can be a global model parameter vector. In particular, the global model update can be determined by aggregating each local update received by the server. In some implementations, the aggregation can include determining a weighted average of the received local updates.

In some implementations, if additional privacy is needed, randomization techniques from differential privacy can be used. The centralized algorithm could be modified to produce a differentially private model, which allows the model to be released while protecting the privacy of the individuals contributing updates to the training process. If protection from even a malicious (or compromised) centralized model trainer is needed, techniques from local differential privacy can be applied to privatize the individual updates.

More particularly, in some embodiments, a set of input-output data can be used to describe a global objective via a loss function. Such functions can be, for instance, a convex or non-convex function, such as a linear regression function, logistic regression function, a support vector machine, neural network function and/or various other suitable functions. A local objective (F_(k)) can also be defined using data stored on a computing device. For instance, the global objective can be defined as:

${f(w)} = {{\sum\limits_{k = 1}^{K}\;{\frac{n_{k}}{n}{F_{k}(w)}}} = {\sum\limits_{k = 1}^{K}\;{{\frac{n_{k}}{n} \cdot \frac{1}{n_{k}}}{\sum\limits_{i \in \mathcal{P}_{k}}\;{f_{i}(w)}}}}}$

wherein K describes the number of computing devices, n describes a total number of data examples, n_(k) describes the number of data examples stored on computing device k, and P_(k) describes a partition of data example indices {1, . . . , n} stored on the computing device k. In this manner, the local objective can specify an optimization problem to be solved by device k. According to particular implementations of the present disclosure, the global objective can be solved by aggregating a plurality of local updates provided by a plurality of remote computing devices. Each remote computing device can, for instance, be a user device, such as a laptop computing device, desktop computing device, smartphone, tablet, wearable computing device, or other computing device. The local updates can be determined based at least in part on the respective local objectives.

In particular, a gradient of the global objective can be determined. For instance, each computing device can be configured to determine the gradient of the local objective using the data examples stored on the computing device. In some implementations, the global gradient may be equal to a weighted sum of the local gradients. In implementations wherein the global gradient and the local gradients are not equal, each remote computing device can be configured to provide the determined gradient to the central computing device (e.g. server device, data center, etc.). The central device can then be configured to determine a gradient of the global objective based at least in part on the local objective gradients, and then to provide the gradient to the remote computing devices. For instance, the gradient can be defined as:

${\nabla{f\left( \overset{\sim}{w} \right)}} = {\frac{1}{n}{\sum\limits_{i = 1}^{n}{\nabla{f_{i}\left( \overset{\sim}{w} \right)}}}}$

Each remote computing device can then determine a local update based at least in part on the global gradient. For instance, the local update can be determined using one or more gradient descent techniques (e.g. stochastic gradient descent). In this manner, each remote computing device can perform one or more stochastic updates or iterations to determine the local update. More particularly, each remote computing device can initialize one or more parameters associated with the local objective. Each remote computing device can then, for instance, uniformly, randomly sample Pk for one or more stochastic iterations. In this manner, the local update can be determined based at least in part on the sampled data. In particular, the local update can be defined as:

-   -   for t=1 to m do         -   Sample i∈             uniformly at random             w _(k) =w _(k) −h(∇f _(i)(w _(k))−∇f _(i)({tilde over (w)}             _(s))+∇f({tilde over (w)} _(s)))             wherein m is a number of stochastic steps per iteration, and             h is the stepsize.

The local updates can then be provided to the central computing device. In particular, the local updates can be gradient vectors. The central computing device can then aggregate the local updates to determine a global update to the model. For instance, the aggregation can be an averaging aggregation defined as:

$\overset{\sim}{w} = {\overset{\sim}{w} + {\frac{1}{K}{\sum\limits_{k = 1}^{K}\left( {w_{k} - \overset{\sim}{w}} \right)}}}$ This can be repeated for one or more iterations, for instance, until the loss function reaches a threshold (e.g. converges). The threshold can be determined based at least in part on a desired accuracy of the global model. As another example, the threshold can be determined based at least in part on the amount of time required for communication between the central computing device and the remote computing devices.

The above process can be modified to achieve good empirical performance in the full federated setting. This setting can come with the data available locally being clustered around specific pattern, and thus not being a representative sample of the overall distribution that the system is attempting to learn.

Note that in large scale generalized linear prediction problems, the data arising are very frequently sparse, for example due to bag-of-words style feature representations. This means that only a small subset of d elements of vector x_(i) have nonzero values. In the case of linear predictors, the gradient is a multiple of the data vector x_(i). This creates additional complications, but also potential for exploitation of the problem structure and thus faster algorithms.

In particular, aspects of the present disclosure provide solutions when encountering a setting with data sparsity. If one looks at how frequently a feature appears in the whole dataset, and find out that locally, the feature appears much more often, the node with such local data is going to overshoot in estimates of gradient in this feature. Simply because the device samples this feature more often than the system would if it were sampling from the whole dataset. For this reason, among others, embodiments of the present disclosure scale local stochastic gradients by ratio of per-feature appearance frequencies globally, and locally.

Some notation is now provided.

n—number of data examples/data points/functions.

—set of indices, corresponding to data points stored on device k.

n_(k)=|

|—number of data points stored on device k.

n^(j)i=|{i∈[n]:

x_(i), e_(j)

}|—the number of data points with nonzero j^(th) coordinate.

n_(k) ^(j)=|{i∈

:

x_(i), e_(j)

≠0}|—the number of data points stored on node k with nonzero j^(th) coordinate.

ϕ^(j)=n^(j)/n—frequency of appearance of nonzero elements in j^(th) coordinate.

ϕ_(k) ^(j)=n_(k) ^(j)/n_(k)—frequency of appearance of nonzero elements in j^(th) coordinate on node k.

s_(k) ^(j)=ϕ^(j)/ϕ_(k) ^(j)—ratio of global and local appearance frequencies on node k in j^(th) coordinate.

S_(k)=Diag(s_(k) ^(j))—diagonal matrix, composed of s_(k) ^(j) as j^(th) diagonal element.

w^(j)=|{

: n_(k) ^(j)≠0}|—Number of nodes that contain data point with nonzero j^(th) coordinate.

α_(j)=K/w^(j)—aggregation parameter for coordinate j.

A=Diag(a^(j))—diagonal matrix composed of α^(j) as j^(th) diagonal element.

In example embodiments, the local objective can further be determined based at least in part on the distribution of data among the remote computing devices. In particular, a local stepsize (h_(k)) associated with the stochastic gradient descent for each remote computing device can be determined based at least in part on the number of data examples stored on the remote computing device (e.g., h_(k)=h/n_(k)). Further, the local objective or stochastic gradient can be scaled using a diagonal matrix (S_(k)). In this manner, the local objective can be defined as:

Let {i_(i)}_(t=1) ^(n) ^(k) be random permutation of

for t=1, . . . , n_(k) do w _(k) =w _(k) −h _(k)(S _(k)[∇f _(i1)(w _(k))−∇f _(i1)({tilde over (w)})]+∇f({tilde over (w)}))

In such embodiments, the aggregation of local updates by the central computing device can be proportional to the number of data examples

$\left( {{e.g.},{\frac{n_{k}}{n}\left( {w_{k} - \overset{\sim}{w}} \right)}} \right).$ The aggregation can further include a per-coordinate scaling of aggregated updates (e.g., A(w_(k)−{tilde over (w)})). In this manner, the global model can be an additive aggregation determined as follows:

$\overset{\sim}{w} = {\overset{\sim}{w} + {A{\sum\limits_{k = 1}^{K}{\frac{n_{k}}{n}\left( {w_{k} - \overset{\sim}{w}} \right)}}}}$

Such embodiments can be determined under the assumption that, as some point in time, each remote computing device has the same local iterate. In this manner, each remote computing device can begin from the same point. A linear term: ∇F _(k)({tilde over (w)} _(s))−∇{tilde over (f)}({tilde over (w)} _(s)) can be added to the local objective to force all nodes to initially move in the same direction.

As an example, the update direction of remote computing device k can be denoted as: G _(k) =∇f _(i)(w)−∇f _(i)({tilde over (w)})+∇f({tilde over (w)}) wherein i is sampled uniformly at random from Pk. If there is only one remote computing device then:

[G ₁]=∇f(w). If there are more than one remote computing device, the values of Gk are biased estimates of the gradient of f(w). In this manner,

[Σ_(k=1) ^(K)α_(k) G _(K)]=∇f(w) can be achieved for some choice of α_(k). This can be motivated by a desire of stochastic first-order methods to perform a gradient step on expectation. In this manner, we have:

${{\mathbb{E}}\left\lbrack {\sum\limits_{k = 1}^{K}\;{\alpha_{k}G_{k}}} \right\rbrack} = {\sum\limits_{k = 1}^{K}{\alpha_{k}\frac{1}{n_{k}}{\sum\limits_{i \in \mathcal{P}_{k}}\;\left\lbrack {{\nabla{f_{i}(w)}} - {\nabla{f_{i}\left( \overset{\sim}{w} \right)}} + {\nabla{f\left( \overset{\sim}{w} \right)}}} \right\rbrack}}}$ By setting α_(k) to nk/n, we get:

${{\mathbb{E}}\left\lbrack {\sum\limits_{k = 1}^{K}\;{\alpha_{k}G_{k}}} \right\rbrack} = {{\frac{1}{n}{\sum\limits_{k = 1}^{K}{\sum\limits_{i \in \mathcal{P}_{k}}\;\left\lbrack {{\nabla{f_{i}(w)}} - {\nabla{f_{i}\left( \overset{\sim}{w} \right)}} + {\nabla{f\left( \overset{\sim}{w} \right)}}} \right\rbrack}}} = {\nabla{f(w)}}}$

This motivates the aggregation of updates from nodes proportional to n_(k). The local stepsize h_(k) can be included, for instance, when the number of data examples on each remote computing device is not equal. In such embodiments, it may not be beneficial to perform the same number of stochastic iterations on each remote device. In this manner, setting the stepsize as inversely proportional to the number of data examples on the remote device can allow each remote device to travel roughly the same distance. In this manner, the local and global model updates can be implemented as follows:

  parameters: h = stepsize, data partition {

 }_(k=1) ^(K), diagonal matrices A, S_(k) ∈

 for k ∈ {1, . . . , K} for s = 0, 1, 2, . . . , do   ${{Compute}\mspace{14mu}{\nabla{f\left( \overset{\sim}{w} \right)}}} = {\frac{1}{n}{\sum_{i = 1}^{n}\;{\nabla{f_{i}\left( \overset{\sim}{w} \right)}}}}$  for k = 1 to K do in parallel over nodes k   Initialize: w_(k) = {tilde over (w)} and h_(k) = h/n_(k)   Let {i_(t)}_(t=1) ^(n) ^(k) be random permutation of  

  for t = 1, . . . , n_(k) do    w_(k) = w_(k) − h_(k) (S_(k) [∇ f_(i) _(t) (w_(k)) − ∇ f_(i) _(t) ({tilde over (w)})] + ∇ f({tilde over (w)}))   end for  end for   $\overset{\sim}{w} = {\overset{\sim}{w} + {A{\sum_{k = 1}^{K}\;{\frac{n_{k}}{n}\left( {w_{k} - \overset{\sim}{w}} \right)}}}}$ end for

In further embodiments, the local objective can be defined as follows:

$w_{k} = {\underset{w \in R^{d}}{\arg\mspace{11mu}\min}{\left\{ {{n_{k}{F_{k}(w)}} + {\left( {{n{\nabla{f\left( {\overset{\sim}{w}}_{s} \right)}}} - {n_{k}{\nabla{F_{k}\left( {\overset{\sim}{w}}_{s} \right)}}}} \right)^{T}w} + {\frac{\beta}{2}{{U^{\frac{1}{2}}\left( {w - {\overset{\sim}{w}}_{s}} \right)}}^{2}}} \right\}.}}$ In such embodiments, a diagonal matrix UΣ

is defined having diagonal entries:

$u_{j} = {\beta\frac{n^{j}}{n_{k}^{j}}}$ A regularizer term is defined as:

$\frac{1}{2}{{{U^{\frac{1}{2}}\left( {w - {\overset{\sim}{w}}_{s}} \right)}}^{2}.}$ The u_(j) can correspond to a curvature in dimension i. For instance, u_(j)=β can be very small when each ith data example is on the current remote computing device. In this manner, w_(j) can be determined as desired by the remote computing device. In such embodiments, when n_(k) ^(j)=0, the regularization is infinitely strong, and the coefficients of features for which no data is present may not be changed.

In some implementations, stochastic gradient descent techniques can be naively applied to the optimization problem, wherein one or more “minibatch” gradient calculations (e.g. using one or more randomly selected use devices) are performed per round of communication. For instance, the minibatch can include at least a subset of the training data stored locally on the user devices. In such implementations, one or more user devices can be configured to determine the average gradient associated with the local training data respectively stored on the user devices for a current version of a model. The user devices can be configured to provide the determined gradients to the server, as part of the local updates. The server can then aggregate the gradients to determine a global model update. As another example, in some implementations, one or more user devices can perform one or more “steps” of gradient descent on the current model using at least a subset of the local training data respectively stored on the user devices, and provide data indicative of the one or more gradient descent steps to the server. The server can then aggregate the data, for instance, by determining a weighted average. For instance, the user devices may determine an updated version of the model (e.g. using one or more stochastic gradient descent techniques) using local data. The server can then determine a weighted average of the resulting models to determine a global update to the model.

With reference now to the figures, example embodiments of the present disclosure will be discussed in further detail. For instance, FIG. 1 depicts an example system 300 for training one or more global machine learning models 306 using training data 308 stored locally on a plurality of user devices 302. System 300 can further include a server device 304. Server 304 can be configured to access machine learning model 306, and to provide model 306 to a plurality of user devices 302. Model 306 can be, for instance, a linear regression model, logistic regression model, a support vector machine model, a neural network (e.g. convolutional neural network, recurrent neural network, etc.), or other suitable model. In some implementations, sever 304 can be configured to communicate with user devices 302 over one or more networks, such as network 240 of FIG. 3.

User devices 302 can each be configured to determine one or more local updates associated with model 306 based at least in part on training data 308. For instance, training data 308 can be data stored locally on the user devices 306. In this manner, training data 308 can include audio files, image files, video files, a typing history, location history, and/or various other suitable data. In some implementations, the training data can be any data derived through a user interaction with a user device 302.

In some implementations, the local updates can be a gradient vector associated with the model. For instance, user devices 302 can determine a gradient (e.g. an average gradient) associated with the model based at least in part on training data 308 respectively stored on user devices 302. In some implementations, the local update can include an updated version of model 306 determined by taking one or more stochastic gradient descent steps. It will be appreciated that the local updates can include any suitable data indicative of one or more local training techniques respectively performed by user devices 302 based at least in part on training data 308. In some implementations, the local updates can include one or more training passes, or epochs. In this manner the minibatch of training data can be applied one or more times to determine the local update.

User devices 302 can then be configured to provide the local updates to server 304. As indicated above, training data 308 may be privacy sensitive. In this manner, the local updates can be performed and provided to server 304 without compromising the privacy of training data 308. For instance, in such implementations, training data 308 is not provided to server 304. The local update does not include training data 308. In implementations, wherein a locally updated model is provided to server 304, privacy sensitive data may be able to be derived or inferred from the model parameters. In such implementations, one or more encryption, random noise techniques, and/or other security techniques can be added to the training process to obscure any inferable information.

As indicated above, server 304 can receive each local update from user device 302, and can aggregate the local updates to determine a global update to the model. In some implementations, server 304 can determine a weighted average of the local updates and determine the global update based at least in part on the average.

In some implementations, scaling or other techniques can be applied to the local updates to determine the global update. For instance, a local stepsize can be applied for each user device 302, the aggregation can be performed proportionally to various data partition sizes of user devices 302, and/or one or more scaling factors can be applied to the local and/or aggregated updates. It will be appreciated that various other techniques can be applied without deviating from the scope of the present disclosure.

FIG. 2 depicts a flow diagram of an example method (100) of determining a global model according to example embodiments of the present disclosure. Method (100) can be implemented by one or more computing devices, such as one or more of the computing devices depicted in FIG. 3. In addition, FIG. 2 depicts steps performed in a particular order for purposes of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, will understand that the steps of any of the methods discussed herein can be adapted, rearranged, expanded, omitted, or modified in various ways without deviating from the scope of the present disclosure.

At (102), method (100) can include determining, by a user device, a local gradient based on one or more local data examples. In particular, the local gradient can be determined for a loss function using the one or more data examples. In some implementations, the user device can be a wearable computing device. The data examples may be generated, for instance, through interaction of a user with the user device. At (104), method (100) can include providing, by the user device, the local gradient to a server, and at (106), method (100) can include receiving, by the server, the local gradient.

At (108), method (100) can include determining, by the server, a global gradient based at least in part on the received local gradient. For instance, the can be determined based at least in part on a plurality of local gradients provided by a plurality of user devices, each having a plurality of unevenly distributed data examples. In particular, the data example may be distributed among the user devices such that no user device includes a representative sample of the overall distribution of data. In addition, the number of user devices may exceed the number of data examples on any one user device.

At (110), method (100) can include providing the global gradient to each user device, and at (112), method (100) can include receiving the global gradient.

At (114), method (100) can include determining, by the user device, a local update. In a particular implementation, the local update can be determined based at least in part on the global update. In other implementations, the global gradient may not be required to determine the local update. For instance, in such implementations, each local gradient may be identical. In such embodiments, the global gradient may not be determined by the server, and method (100) can bypass (104)-(112).

The local update can be determined based at least in part using one or more stochastic updates or iterations. For instance, the user device may randomly sample a partition of data examples stored on the user device to determine the local update. In particular, the local update may be determined using stochastic gradient descent techniques to determine a direction in which to adjust one or more parameters of the loss function.

In some embodiments, a stepsize associated with the local update determination can be determined based at least in part on a number of data examples stored on the user device. In further embodiments, the stochastic gradient can be scaled using a diagonal matrix, or other scaling technique. In still further embodiments, the local update can be determined using a linear term that forces each user device to update the parameters of the loss function in the same direction.

At (116), method (100) can include providing, by the user device, the local update to the server, and at (118), method (100) can include receiving, by the server, the local update. In particular, the server can receive a plurality of local updates from a plurality of user devices.

At (120), method (100) can include determining a global model. In particular, the global model can be determined based at least in part on the received local update(s). For instance, the received local updates can be aggregated to determine the global model. The aggregation can be an additive aggregation and/or an averaging aggregation. In particular implementations, the aggregation of the local updates can be proportional to the partition sizes of the data examples on the user devices. In further embodiments the aggregation of the local updates can be scaled on a per-coordinate basis. Any number of iterations of local and global updates can be performed.

FIG. 3 depicts an example computing system 200 that can be used to implement the methods and systems according to example aspects of the present disclosure. The system 200 can be implemented using a client-server architecture that includes a server 210 that communicates with one or more client devices 230 over a network 240.

The system 200 includes a server 210, such as a web server. The server 210 can be implemented using any suitable computing device(s), and can correspond to server 304 of FIG. 1. The server 210 can have one or more processors 212 and one or more memory devices 214. The server 210 can also include a network interface used to communicate with one or more client devices 230 over the network 240. The network interface can include any suitable components for interfacing with one more networks, including for example, transmitters, receivers, ports, controllers, antennas, or other suitable components.

The one or more processors 212 can include any suitable processing device, such as a microprocessor, microcontroller, integrated circuit, logic device, or other suitable processing device. The one or more memory devices 214 can include one or more computer-readable media, including, but not limited to, non-transitory computer-readable media, RAM, ROM, hard drives, flash drives, or other memory devices. The one or more memory devices 214 can store information accessible by the one or more processors 212, including computer-readable instructions 216 that can be executed by the one or more processors 212. The instructions 216 can be any set of instructions that when executed by the one or more processors 212, cause the one or more processors 212 to perform operations. For instance, the instructions 216 can be executed by the one or more processors 212 to implement global updater 220. The global updater 220 can be configured to receive one or more local updates and to determine a global model based at least in part on the local updates.

As shown in FIG. 3, the one or more memory devices 214 can also store data 218 that can be retrieved, manipulated, created, or stored by the one or more processors 212. The data 218 can include, for instance, local updates, global parameters, and other data. The data 218 can be stored in one or more databases. The one or more databases can be connected to the server 210 by a high bandwidth LAN or WAN, or can also be connected to server 210 through network 240. The one or more databases can be split up so that they are located in multiple locales.

The server 210 can exchange data with one or more client devices 230 over the network 240. Any number of client devices 230 can be connected to the server 210 over the network 240. Each of the client devices 230 can be any suitable type of computing device, such as a general purpose computer, special purpose computer, laptop, desktop, mobile device, navigation system, smartphone, tablet, wearable computing device, a display with one or more processors, or other suitable computing device. In some implementations, the client devices 230 can correspond to user devices 302 of FIG. 1.

Similar to the server 210, a client device 230 can include one or more processor(s) 232 and a memory 234. The one or more processor(s) 232 can include one or more central processing units (CPUs), graphics processing units (GPUs) dedicated to efficiently rendering images or performing other specialized calculations, and/or other processing devices. The memory 234 can include one or more computer-readable media and can store information accessible by the one or more processors 232, including instructions 236 that can be executed by the one or more processors 232 and data 238. The data 238 can include one or more data examples to be used in solving one or more optimization problems. The data examples of each client device 230 can be distributed unevenly among the client devices, such that no client device 230 includes a representative sample of the overall distribution of the data examples. The memory 234 can store instructions 236 for implementing a local updater configured to determine one or more local updates according to example aspects of the present disclosure.

The client device 230 of FIG. 3 can include various input/output devices for providing and receiving information from a user, such as a touch screen, touch pad, data entry keys, speakers, and/or a microphone suitable for voice recognition.

The client device 230 can also include a network interface used to communicate with one or more remote computing devices (e.g. server 210) over the network 240. The network interface can include any suitable components for interfacing with one more networks, including for example, transmitters, receivers, ports, controllers, antennas, or other suitable components.

The network 240 can be any type of communications network, such as a local area network (e.g. intranet), wide area network (e.g. Internet), cellular network, or some combination thereof. The network 240 can also include a direct connection between a client device 230 and the server 210. In general, communication between the server 210 and a client device 230 can be carried via network interface using any type of wired and/or wireless connection, using a variety of communication protocols (e.g. TCP/IP, HTTP, SMTP, FTP), encodings or formats (e.g. HTML, XML), and/or protection schemes (e.g. VPN, secure HTTP, SSL).

The technology discussed herein makes reference to servers, databases, software applications, and other computer-based systems, as well as actions taken and information sent to and from such systems. One of ordinary skill in the art will recognize that the inherent flexibility of computer-based systems allows for a great variety of possible configurations, combinations, and divisions of tasks and functionality between and among components. For instance, server processes discussed herein may be implemented using a single server or multiple servers working in combination. Databases and applications may be implemented on a single system or distributed across multiple systems. Distributed components may operate sequentially or in parallel.

While the present subject matter has been described in detail with respect to specific example embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art. 

What is claimed is:
 1. A computer-implemented method of updating a global model based on unevenly distributed data, the method comprising: providing, by one or more computing devices, a current global model to a plurality of user devices; receiving, by the one or more computing devices, a plurality of local updates to the current global model from the plurality of user devices, each local update being determined by the respective user device through performance of a respective number of iterations of a gradient descent training technique on the current global model with respect to a respective number of data examples stored on the respective user device, and wherein the one or more data examples stored on the plurality of user devices are distributed on an uneven basis, such that no user device includes a representative sample of an overall distribution of data examples; and aggregating, by the one or more computing devices, the received local updates to determine an updated global model, wherein aggregating, by the one or more computing devices, the received local updates to determine the updated global model comprises applying, by the one or more computing devices, a respective weighting term to each local update, and wherein the respective weighting term for each local update is proportional to the respective number of data examples stored on the user device from which such local update was received.
 2. The computer-implemented method of claim 1, further comprising transmitting, by the one or more computing devices, the updated global model to the plurality of user devices for use at the plurality of user devices in generating inferences.
 3. The computer-implemented method of claim 1, wherein at least one of the local updates comprise a gradient vector obtained through performance of the respective number of iterations of the gradient descent training technique on the current global model with respect to the number of data examples stored on the respective user device.
 4. The computer-implemented method of claim 1, wherein the size of each local update is independent from the size of the data used to determine the local update.
 5. The computer-implemented method of claim 1, wherein, for each of the respective number of training iterations, the respective user device employs a respective device-specific stepsize that controls an amount of change to one or more parameters of the current global model at each iteration, wherein, for each respective user device, the respective device-specific stepsize is inversely proportional to the number of data examples stored on the respective user device.
 6. The computer-implemented method of claim 1, wherein, for each respective user device, the respective number of iterations of the gradient descent training technique are determined at least in part by randomly sampling the data examples stored on the respective user device.
 7. The computer-implemented method of claim 1, wherein the one or more local updates are determined by applying, by each user device, a respective device-specific diagonal scaling matrix, wherein the device-specific diagonal scaling matrix for each user device describes, on a coordinate-by-coordinate basis, a ratio of a global appearance frequency of the coordinate to a local appearance frequency of the coordinate on the user device.
 8. The computer-implemented method of claim 1, wherein aggregating, by the one or more computing devices, the received local updates to determine the updated global model comprises scaling the received local updates on a per-coordinate basis using, by the one or more computing devices, a diagonal matrix that describes, on a coordinate-by-coordinate basis, a ratio of the number of user devices to a number of user devices that contain at least one datapoint that is non-zero for such coordinate.
 9. The computer-implemented method of claim 1, wherein aggregating, by the one or more computing devices, the received local updates to determine the updated global model comprises aggregating the received local updates for at least one iteration.
 10. The computer-implemented method of claim 9, wherein the at least one iteration is determined based at least in part on a threshold.
 11. The computer-implemented method of claim 10, wherein the threshold is determined based at least in part on an amount of time required for communication of the one or more local updates.
 12. The computer-implemented method of claim 1, wherein the number of data examples stored on each user device is smaller than a total number of user devices.
 13. A computer-implemented method of updating a local machine learning model based on unevenly distributed data, the method comprising: determining, by a user device, a local model update based at least in part on a gradient vector of a loss function and a number of locally stored data examples, wherein the distribution of the number of locally stored data examples is not representative of an overall distribution of data examples used to train a global machine learning model; providing, by the user device, the local model update to a central computing device for use in determination of an update to the global machine learning model, the update to the global machine learning model being determined based on aggregation of the local model update with one or more additional local model updates received from one or more additional user devices, the aggregation of the local model update with the one or more additional local model updates comprising application of a weighting term to the local model update, the weighting term for the local model update being proportional to the number of locally stored data examples stored on the user device; after determination of the update to the global machine learning model, receiving, by the user device, the global machine learning model from the central computing device; and employing, by the user device, the global machine learning model to produce predictions.
 14. The computer-implemented method of claim 13, wherein determining, by the user device, the local model update comprises applying, by the user device, a device-specific diagonal scaling matrix that describes, on a coordinate-by-coordinate basis, a ratio of a global appearance frequency of the coordinate to a local appearance frequency of the coordinate.
 15. The computer-implemented method of claim 13, wherein determining, by the one or more computing devices, the local model update comprises determining the local model update based at least in part on one or more stochastic iterations, each stochastic iteration having a device-specific stepsize that is inversely proportional to the number of the locally stored data examples stored on the user device.
 16. The computer-implemented method of claim 15, wherein the one or more stochastic iterations are determined at least in part by randomly sampling the locally stored data examples stored on the user device.
 17. A computing system, comprising: one or more processors; and one or more memory devices, the one or more memory devices storing computer-readable instructions that when executed by the one or more processors cause the one or more processors to perform operations, the operations comprising: determining a local model update associated with an objective function based at least in part on a number of local data examples stored by the computing system, the local model update being determined by performing a number of iterations of a gradient descent training technique on a local version of a model with respect to the number of local data examples stored by the computing system; providing the local model update to a central computing device for use in the determination of a global version of the model based on aggregation of the local model update with one or more additional local model updates received from one or more additional computing systems, the aggregation of the local model update with the one or more additional local model updates comprising application of a weighting term to the local model update, the weighting term for the local model update being proportional to the number of local data examples stored on the computing system; receiving the global version of the model from the central computing device; and employing the global version of the model to produce predictions; wherein a distribution of the number of local data examples is not representative of an overall distribution of data examples stored on the one or more additional computing systems.
 18. The computing system of claim 17, wherein performing the number of iterations of the gradient descent training technique on the local version of the model comprises performing the number of iterations of the gradient descent training technique on a number of minibatches of the local data examples stored by the computing system, wherein the number of minibatches equals the number of iterations, and wherein the number of iterations comprises two or more iterations such that the computing system performs two or more gradient descent steps per round of communication with the central computing device.
 19. The computing system of claim 17, wherein performing the number of iterations of the gradient descent training technique on the local version of the model comprises performing the number of iterations of the gradient descent training technique on the number the local data examples stored by the computing system, wherein the number of the local data examples equals the number of iterations such that one gradient descent step is performed for each of the local data examples.
 20. The computing system of claim 17, further comprising the central computing device, wherein the central computing device aggregates the local model update with one or more additional local model updates received from one or more additional devices, and wherein to aggregate the local model update with the one or more additional local model updates the central computing applies a diagonal aggregation matrix that describes, on a coordinate-by-coordinate basis, a ratio of a number of participating devices to a number of devices that contain at least one datapoint that is non-zero for such coordinate. 